1. Field of the Invention
The present invention relates to a pneumatic tire having an improved belt structure being capable of decreasing the tire weight without decreasing the tire strength and the belt hooping effect.
2. Description of the Related Art
As an approach to the environmental problems such as global warming and air pollution, it is a main theme to improve automobiles' fuel consumption. For the tires, therefore, it is very important to decrease the weight, which is especially true of pneumatic tires for heavy duty vehicles such as trucks and buses.
Nowadays, radial tires are widely used, and the radial tires are provided radially outside the carcass (a) with a breaker (b) as shown in FIG. 10 to reinforce the tread portion. In general, the breaker for heavy duty radial tires is composed of four cut-end plies (b1-b4) of rubberized parallel cords (c) to provide a necessary strength for the tread portion. If the number of the cut-end plies is decreased, it is difficult for the tire to pass a strength test. Here, the strength (hereinafter, plunger strength) corresponds to the so called plunger energy obtained by the method specified in Japanese Industrial Standard D4230.
On the other hands, if the inclination angle of the breaker cords to the tire equator C is increased, the envelope effect is enhanced at the sacrifice of the hooping effect. Therefore, there are some instances where the necessary plunger strength can be barely obtained by increasing the inclination angle even if the breaker ply number is decreased. In this case, however, a tire growth, especially, a partial growth greater in the tread crown portion is inevitable because of the decreased hooping effect.